This invention generally relates to signal processing applications. More particularly, the present invention relates to a system and method for an adaptive noise reducer for reducing MPEG noise in a digital video signal.
Video signals may be corrupted by many forms of noise that are attributable to multiple sources. One source of noise in video signals is the compression of the video signal.
When the video information is stored on a digital video disc (DVD) or other digital media it is typically stored in a compressed format. Ninety minutes of uncompressed standard-definition video stored in a uncompressed format (Y:Cb:Cr 4:2:2) will require approximately 90 gigabytes (GB) of storage space. Uncompressed high-definition video of the same length would require six times the amount of storage space, or 540 GB. However, typical DVD disks only have capacities of approximately 4.7 gigabytes. Therefore storing a full-length standard-definition movie on a single DVD disk requires the video to be compressed using a format having a compression ratio on the order of 20:1. For high-definition content, higher capacity disks are used with similar compression ratios (e.g., approximately 30 GB are required to store a full-length movie in Blu-Ray format). In order to stream high-quality video (e.g., over the Internet), compression ratios as high as 50:1 may be required (depending on channel bandwidth).
The MPEG formats are a set of standards established for the compression of digital video and audio data. Where predictive coding of motion pictures is used, as in MPEG-1, compression artifacts tend to remain on several generations of decompressed frames, leading to a “painting” effect being seen, as if the picture were being painted by an unseen artist's paint-brush. Where motion prediction is used, as in MPEG-2 or MPEG-4, compression artifacts tend to move with the optic flow of the image, leading to a peculiar effect, part way between a painting effect and “grime” that moves with objects in the scene. Errors in the bit-stream can lead to errors similar to large quantization errors, or can disrupt the parsing of the data stream entirely for a short time, leading to “break-up” of the picture. Where gross errors have occurred in the bit-stream, it is not uncommon for decoders to continue to apply “painting” updates to the damaged picture, creating “ghost image” effects.
To stop the build-up of compression artifacts, most compression systems occasionally send an entire compressed frame without prediction or differencing, usually at the start of a shot and at regular intervals thereafter. In MPEG picture coding, these are known as “I-frames”, with the ‘I’ standing for “intraframe compression.”
Various techniques have been used to attempt to address the noise that occurs in a digital video signal. With digital displays becoming bigger, it becomes even more important to remove noise. Larger displays will make the noise more readily discernable to a viewer of the video. Many of the techniques previously applied are incomplete, resulting in a video signal that has its noise effects only marginally improved.